What is the lesson in the story that the author is trying to teach the reader?

THis is really a thematic question. I think loss of innicence is what O'Brien is really trying to illustrate. The Vietnam War both defiles and preserves the innocence of those who participate. Most of the men at war are young, not yet twenty. Many have peachy skin and blonde hair. But O'Brien is relentless about pointing out that although they are young and innocent, they are killers. They kill on command, which makes their crimes seem somehow mitigated. But there is also the sense that they are in a primordial jungle of sorts and are somehow inventing evil anew each day. Perhaps the clearest parable of the loss of innocence is the story "The Sweetheart of the Song Tra Bong", in which a young blonde soldier's girlfriend is brought over from America only to become intoxicated with Vietnam and become a killer.